The present invention relates to a rotation angle detecting device to be used in an electric power steering device for a vehicle, for detecting the angle of rotation of a rotary member, and to a torque detecting device for detecting a torque to be applied to the rotary member.
As an auxiliary steering device mounted on a vehicle such as an automobile for assisting the steering operation of a driver, there is an electric power steering device for applying a steering assisting force such as the turning force of an electric motor. This electric power steering device is provided with an input shaft and an output shaft, which are so connected to a steering member and steering wheels sides respectively, as to rotate according to the steering operation of the driver. The steering device is provided with a rotation angle detecting device for detecting the respective rotation angles of the input and output shafts, and a torque detecting device for detecting the steering torque to be applied to the steering member by using the detection results of the detecting device. The steering device assists the steering operation by deciding an instruction value to the electric motor on the basis of the detected steering torque and by transmitting the motor turning force to a steering system through a reduction mechanism thereby apply the steering assisting force to the steering system (as referred to JP-A-2002-107112, for example).
Here, the rotation angle detecting device and the torque detecting device are provided with: a target so fixedly fitted on each of the input and output shafts as to rotate together and having a plurality of teeth made of a magnetic material; and a magnetic sensor including magnetoresistive elements for outputting output signals varying periodically according to the rotations of the corresponding input and output shafts. In the devices, the rotation angle can be detected based on the output signals (or its digitized signals, if necessary) from the magnetic sensor and by referring to a table, which is stored in advance with the rotation angle and the output signals of the magnetic sensor in a corresponding manner. On the other hand, the torque can be detected by determining the rotation angle difference (or the relative angle displacement) between the input shaft and the output shaft using the output signal (or its digitized signal, if necessary) from the magnetic sensor on the input shaft side and the output signal (or its digitized signal, if necessary) from the magnetic sensor on the output shaft side, and by calculating the relative angle displacement. In these devices, moreover, the target or the objective to be detected by the magnetic sensor uses a spur gear 54, in which the side faces 51 of a tooth are formed into an involute curve (as seen in a top plan view, as in the following) and in which the two end portions 53 of a tooth crest 52 are formed into a smooth curve.
However, the involute-shaped gear 54 generally has a main object to effect the power transmission by a meshing engagement and is worked to form the two end portions 53 of the tooth crest 52 into the gentle curve having no angular portion because it is intended to eliminate a failure such as chip. Therefore, a problem is that it is difficult to make the shapes of all teeth identical in an excellent size precision. On the other hand, the output signals from the magnetic sensors are decided mainly by the distance from the tooth crest 52 so that the target is generally tested with the tooth pitch. In the case of the involute gear 54, however, it is necessary to measure the tooth pitch L, as shown in FIG. 7, by supporting virtual corner portions 61. This necessity raises a problem that the target or the objective of the sensor detection is hard to test.